Cracking WPA2-PSK with Aircrack

This article is an excerpt from my WiFi Penetration testing and Security eBook for aspiring WiFi hackers and Wireless security enthusiasts. Click here to learn more

In this chapter we will cover:

• Intro to WPA2
• What is Dictionary attack ?
• Capturing WPA2 handshake
• Aireplay-ng
• Cracking
• Conclusion

In the previous chapter we learned

• How to crack WEP Encrypted networks using Aircrack-ng

Which left us with an obvious question, How to secure it ? use WPA2-PSK.

WPA2-PSK, WiFi Protected Access – Pre Shared Key, is by far one of the most secure and unbroken wireless security encryption at this moment. There is no encryption flaw yet reported by security researchers for WPA2, so that a malicious hacker can easily take advantage of and easily decrypt packets.

Encryption might be the most secured and unbroken at this point, but WPA2 system is still pretty vulnerable to the hackers.

Unlike WEP, WPA2 uses a 4-way handshake as an authentication process. In which the key is never transmitted over the network but used to encrypt/decrypt the data packets across the network. That allows a hacker to just capture the handshake and perform the attack without Live capturing data packets as we did while cracking WEP.

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Just like the broadcast packets we saw in the previous chapter using wireshark, the 4-way handshake is also in plain text. Which allows a potential hacker to capture the plaintext information like

• Access point MAC address
• Client MAC address
• ESSID – AP Name

Information above is used by the hacker to perform a dictionary attack on the captured 4-way handshake (PCAP File). Let’s see

• What is a dictionary attack ?
• How to perform dictionary attack on WPA2-PSK

What is a dictionary attack ?

Hashing is one of the keys used in the security field professional to protect the users from the malicious attackers.

Hash is simply a cryptographic function that converts a data or file of an arbitrary length or size to a fixed length, which is considered practically impossible to invert or reversed, as no key is involved in the process.

A Hash is always unique

In a dictionary attack,

1. We create/use a wordlist (text file of possible passwords)
2. Take a word at a moment from the wordlist
3. Create its hash using the Hash function, PBKDF2 for WPA2
4. Compare the output value with the existing hash.
5. If value matches, password taken from the wordlist is the correct password

Above steps are involved in the WPA2 passphrase cracking process.

Let’s begin,

Step 1: Start monitor mode

$ ifconfig wlan1             #Check whether card is detected
$ sudo airmon-ng check kill  #Kill process causing issues

• sudo airmon-ng start wlan1   #Start monitor mode

Final output should look like this:

Step 2: Start capture, airodump-ng

We will now start airodump-ng to sniff the air and wait until the desired AP and corresponding client are displayed.

• airodump-ng wlan1mon

As you can see in the above image, “rootsh3ll” is the victim AP . We will now note the information highlighted

• AP (ESSID): rootsh3ll
• AP MAC (BSSID): 64:66:B3:6E:B0:8A
• Client MAC: 30:A8:DB:C6:88:13
• Channel: 11

Hit CTRL-C, and kill airodump-ng.

Now, we will start airodump-ng exclusively to capture packets associated with “rootsh3ll” and save the 4-way handshake in a PCAP file, say rootsh3ll

Step 3: Start airodump-ng exclusively

• airodump-ng –bssid 64:66:B3:6E:B0:8A -c 11 wlan1mon -w rootsh3ll

Here “rootsh3ll” is the output filename provided to the -w parameter

Step 4: Disconnect the client with aireplay-ng.

Now, are two ways for capturing the handshake,

• Wait for a client to connect.
• Disconnect the already connected client.

First option seems to be slow, time taking. Whether in our case, option 2 is just perfect as we have a client connected to the wireless AP “rootsh3ll”.

How does that work ? we use a utility from the aircrack-ng suite named aireplay-ng which allows us to craft and send  a disconnect request to the desired AP with the information we noted down earlier.

We are actually abusing a legitimate Windows(or any other OS) feature. Which forces the wireless card to re connect to the AP when available.

In the second option we are actually making sure that option 1 happens, so that we can capture the handshake.

1. Client disconnects when receives the disconnect packet.
2. Reconnect to the AP
3. 4-way handshake between AP and client
4. Hacker captures the 4-way handshake

let’s disconnect the client now,

Open a new Terminal window and type:

• aireplay-ng –deauth 5 -a 64:66:B3:6E:B0:8A wlan1mon

parameters applied:

–deauth 5: 5 deauth requests broadcasted with BSSID “rootsh3ll”, 0 for endless

-a: parameter to tell aireplay-ng the BSSID

wlan1mon: monitor mode interface

Step 5: Capture the handshake

Meanwhile in the terminal window of airodump-ng, you would notice the top of the output. which says

• WPA Handshake: 64:66:B3:6E:B0:8A

Which simply means that the WPA handshake has been capture for the specific BSSID, which is the AP MAC of rootsh3ll.

Hit CTRL-C, as the handshake has been captured, we will now crack the password using the captured handshake

Step 6: How does a Handshake looks like? Open Wireshark (Optional)

This step is optional, you can open the PCAP file(rootsh3ll-01.cap) in Wireshark for manual inspection, or to see how does a handshake looks like.

Type in terminal wireshark [.cap file], which in our case is

• wireshark rootsh3ll-01.cap
• Type “eapol” in the filter field, press [ENTER]

You would notice the last column, “Info” is showing a message no. from 1 to 4.

This is the 4 way handshake happened during the capture. It is like AP and Client are talking to each other. Notice the Source and Destination tab.

moving on to the next step,

Step 7: Cracking

Here’s an ugly truth

WPA2 password cracking is not deterministic like WEP, because it is based on a dictionary of possible words and we do not know whether the passphrase is in the dictionary or not. So you are never sure whether a specific dictionary will just work or not.

For this tutorial I have beautifully crafted a wordlist, just to demonstrate how the output of the cracked password would look like. Command and the wordlist looks like this:

Here I have directly saved the password in the wordlist to demonstrate how the output would look like, your will be different, obviously.

let’s fire up aircrack-ng and crack the key

Type in terminal, aircrack-ng [.cap file] -w [wordlist], which in our case looks like

• aircrack-ng rootsh3ll-01.cap -w dict

and aircrack-ng has cracked the password in one go.

This is quite odd to see the cracked passphrase for the first time, right ?

Here’s a sample output of the running process, yours would look like the same during the cracking process.

Countermeasures

Use a strong password to stay safe,

Example: Myp@sword8@#,  is a strong password

As it has

• No order in plain English language
• 13 Character password, very secured
• Alpha-numeric and special characters in one makes a very strong password.
• Upper and Lower-case characters.
• No pattern
• Not a mobile number, as mobile numbers can be easily guessed.

Or you can just keep a password with some special characters, a word that isn’t a pattern or a dictionary word.

That will also be good and secured.

Conclusion

We learned the process involved in WPA cracking.

Here is a list of commands we went through the capture and the cracking process

$ ifconfig wlan1 # check wireless IFace
$ sudo airmon-ng check kill  # kill issue causing processes
$ sudo airmon-ng start wlan1 # start monitor mode
$ sudo airodump-ng wlan1mon  # start capturing
$ sudo airodump-ng --bssid 64:66:B3:6E:B0:8A -c 11 wlan1mon -w rootsh3ll # airodump-ng on rootsh3ll
$ sudo aireplay-ng --deauth 5 -a 64:66:B3:6E:B0:8A wlan1mon   # deauthenticate the client
$ sudo aircrack-ng rootsh3ll-01.cap dict # crack the passphrase

That’s all for WPA2 for now, Hope you enjoyed through the chapter.